1. Field of the Invention
This invention relates to novel ester-type liquid crystal compounds, liquid crystal compositions containing the same liquid crystal compounds, and a liquid crystal display device provided with the same liquid crystal composition as a liquid crystal layer.
2. Description of the Prior Art
Heretofore, various display principles have been applied to electrooptical display devices using liquid crystals and practical liquid crystal devices have been already commercially available. Among others, TN type liquid crystal display devices using nematic liquid crystals with a twisting alignment have been widely utilized in wrist watches, table-type electronic computers, etc.
As to liquid crystal display devices having a desirable display system of higher precision and larger area to meet versatility of information, on the other hand, a thermally addressed system using a smectic liquid crystal to make display by mutual action between heat and electric field has been under study. Particularly, it has been regarded as very important to drive the thermally addressed devices using the smectic liquid crystal under a low potential with a low power in the same manner as TN-type liquid crystal display devices using the nematic liquid crystal, and it has been keenly desired to find liquid crystal compounds suitable for that purpose.
Generally, to lower the threshold potential (Vth) of liquid crystal corresponding to the working potential of a liquid crystal display device, it is necessary to increase a dielectric constant anisotropy (.DELTA..sub..epsilon.) of the liquid crystal.
Known typical smectic liquid crystals having a relatively large, positive dielectric constant anisotropy are, for example, liquid crystal compounds represented by the following general formulae II and III: ##STR2## wherein R' is a straight alkyl group having 8 to 12 carbon atoms, and ##STR3## wherein R'O-- is a straight alkoxy group having 8 to 12 carbon atoms, both of which will be hereinafter referred to "conventional type". However, the biphenyl-based liquid crystal compounds have such a disadvantage that the dielectric constant anisotropy as a physical property of liquid crystals is unsatisfactorily small.